Java递归练习-将字符串中的“x”全部移到这个字符串最后

Given a string, compute recursively a new string where all the lowercase 'x' chars have been moved to the end of the string.  endX("xxre") → "rexx" endX("xxhixx") → "hixxxx" endX("xhixhix") → "hihixxx" 题目是codingbat上看到的

自己做的答案
public String endX(String str) {

int index=str.indexOf("x");
int length=str.length();
String newX="";

if(index+1==length){ return str;}
else if(index==0&&(length>1)) {newX=str.substring(index+1,length) ;}
else if(index==-1) return str;
else{
newX=str.substring(0,index)+str.substring(index+1,str.length());}

return endX(newX)+"x";
}